Power regeneration technologies are used to improve the efficiency of hydraulic systems for hydraulic working machines. About such hydraulic systems for hydraulic working machines, a description will be made using, as an example, the hydraulic excavator disclosed in Patent Document 1.
In Patent Document 1, the hydraulic excavator has a configuration that two hydraulic pump motors driven by an electric motor are connected to two ports of a double-acting hydraulic cylinder, respectively. The double-acting hydraulic cylinder is of a single rod type, and the pressure-receiving area of its piston is different between an extension side and a retraction side. Therefore, the displacements of the two hydraulic pump motors are set at a ratio corresponding to the pressure-receiving areas of the piston. To control the speed and direction of the hydraulic cylinder, a controller performs, based on a manipulation stroke of a control lever, to control the rotation speed and rotation direction of the electric motor that drives the hydraulic pump motors. Further, in parallel to a line that connects a bottom side of the hydraulic cylinder and its corresponding hydraulic pump motor together, a line is arranged passing through a spool-type flow rate control valve controllable by the controller. The flow rate control valve is controlled to allow hydraulic oil, which has been discharged from the hydraulic cylinder, to pass through the flow rate control valve in a fine control range that the manipulation stroke of the control lever is smaller than a predetermined value, but is controlled to allow the hydraulic oil, which has been discharged from the hydraulic cylinder, to flow directly into the corresponding hydraulic pump motor without passing through the flow rate control valve when the manipulation stroke of the control lever exceeds the predetermined value. Owing to the configuration as described above, the flow rate control valve assures good speed control performance for the hydraulic cylinder in the fine control range, and the direct connection to the hydraulic pump motor assures good power regeneration efficiency when the manipulation stroke of the control lever exceeds the fine control range.